The Federal Communications Commission governs the use of the limited radio-frequency spectrum made by various industries. The small portion of the spectrum assigned to each industry must be used efficiently to maximize the number of users of the limited spectrum. Accordingly, various multiple access modulation techniques have been developed to fully exploit the available spectrum. For example, some wireless communication systems employ Code Division Multiple Access (CDMA) modulation which uses a spread spectrum technique for information transmission. More specifically, a spread spectrum system uses a modulation method which distributes or spreads the transmitted signal over a wide frequency band, typically much greater than the bandwidth required to transmit the signal. This modulation technique modulates each baseband data signal with a unique wideband spreading code. As a result, a type of code diversity is obtained. Since only a few hundred kilohertz of a signal is typically affected by frequency selective fade, the remaining content of the signal is received substantially as transmitted.
In a conventional CDMA system, multiple signals are transmitted simultaneously at the same frequency. A receiver, such as a portable communications device, determines which signal is intended for that receiver by deciphering the spreading code in the signal. The other signals at that frequency appear as noise to the device, and are ignored. While this technique makes efficient use of the available frequency spectrum, it also places a premium on optimum power control of the signals output by the system components (i.e., the base stations and portable communication devices). High power signals increase the interference experienced by other components in the system, thereby lowering the system capacity for reliable information transmission. Thus, a method for controlling the power output for each component is required to prevent transmission of unnecessarily powerful signals.
In a CDMA system, the portable communication devices assist the base stations in controlling the power on the forward link (from the base station to the portable device) by transmitting a power control signal on the reverse link (from the portable device to the base station). In general, the portable device measures its error performance and provides this information to the base station with the power control signal. The base station then makes any necessary adjustments to the power level transmitted to the portable device to ensure quality information transmission at a minimum required power level.
CDMA communication systems operating under the IS-2000 standard perform forward power link control for the forward fundamental traffic channel (F-FCH) and the dedicated control channel (DCCH) when operating in radio configurations three through nine. Accordingly, the portable device must transmit forward power control bits (FPCs) from which the base station determines the appropriate power allocation to the relevant F-FCH or DCCH.
The IS-2000 standard requires that power control be implemented in a nested, closed-loop control system. The set point for the outer loop of the system is the expected frame error rate (FER) on the received channel (the F-FCH or the DCCH). The outer loop outputs the set point for the ratio of combined received energy per information bit to the effective noise power spectral density (Eb/Nt) for the inner loop. The inner loop estimates the received Eb/Nt over a Power Control Group (PCG) having a 1.25 ms duration, and compares the estimate to the Eb/Nt set point. The result of this comparison is an up or down power control command bit on the reverse link power control subchannel of the reverse link pilot. Accordingly, such systems must estimate the received Eb/Nt over a 1.25 ms duration PCG with sufficient accuracy to permit convergence of the twin closed-loop forward power control system.
The received interference (Nt) may be generated from any of a variety of sources, including multipath interference from all channels due to a loss of Walsh code orthogonality, co-channel interference from nearby cells, receiver noise, or quadrature rotation due to channel estimation error. Nt may be estimated by estimating the variance of a known signal, or estimating the energy on an irrelevant (i.e., not carrying signal information) component (if available) of the received signal. In an IS-2000 system, the known signals are the pilot signals from the same base stations providing the F-FCH and DCCH to the portable device, when received using a receiver with the same combining ratios. Alternatively, an irrelevant component of the received signal is available in IS-2000 systems for the PCBs since they are always transmitted in identical pairs, thereby reducing one degree of freedom from the signal that is otherwise available to the receiver.
The received signal power (Eb), on the other hand, is estimated by measuring the power of the punctured power control symbols since the PCBs are always sent at full rate power. Traffic symbols, however, are not used to estimate the received signal power because traffic has a variable rate which is unknown at the time of estimating Eb/Nt, thereby preventing an accurate determination of whether a reduction of received signal power is due to an actual power drop, or caused by a decreased rate (i.e., redundant symbols sent at a lower level).
Conventional portable devices employ pilot combining to estimate Eb/Nt as further described in “Generalized Eb/Nt Estimation for IS-2000,” Version 1.0, by John Reagan, published Mar. 2, 2000 which is hereby expressly incorporated herein by reference. Pilot symbols may be used for interference estimation only if combined in the same manner as symbols on the traffic channels. Thus, the hardware of each finger element of the rake receiver must perform a complex multiplication (i.e., two-element dot product), resulting in added complexity. Additionally, the device firmware must combine the pilot signal from samples obtained from each finger element using Maximal Ratio Combining (MRC). The process and algorithms for carrying out this estimation are well known in the art.
These systems take advantage of the fact that the value of the combined pilot signal is always positive, conforming to the equation I=Q=+p√{square root over (2)}. Since the pilot signal always lies along the I-Q axis and is nominally of constant amplitude, p, the variance along this axis yields the noise variance. Such systems are further deficient, however, because they require the presence of a pilot signal.
Some conventional systems for estimating Eb/Nt use data bits which were modulated at the base station using quadrature phase shift keying (QPSK). As is well known in the art, QPSK symbols have four values, each located in a separate quadrant of an I-Q plot. Systems such as that described in U.S. Pat. No. 6,154,659, issued Nov. 28, 2000 to Jalali, et al., are deficient because they depend on deciding on the intended transmitted signal prior to estimating Eb/Nt. In reality, however, the intended signal is unknown and such symbol-by-symbol hard decision made without using frame-based coding information is likely to produce a high decision error rate leading to a low overall Eb/Nt estimation accuracy. Furthermore, the noise components of QPSK symbols are more likely to cause the symbols to cross quadrant boundaries because the distances between the noiseless symbols and the decision boundaries are shorter. Also, since all four quadrants contain information, more possibilities for boundary crossings exist, resulting in inaccurate noise estimates.